I would think that the students who are under the age of 17 wouldn't be held accountable for their actions,
and the students who were 17, well, it's not like they were death eaters. They were just expressing their opinion.
And probably punishment would be according to what they had done.
Like, if one student managed to kill someone they would probably be severely punished.
If they managed to give someone else a bruise, who would care?
I hope I make sense here... xD